# DC Nonexistent Peak at Transmitter

**URL:** <https://www.ruby-forum.com/t/dc-nonexistent-peak-at-transmitter/237258>\
**Category:** GNU Radio\
**Created:** [November 1, 2013, 4:25pm UTC](https://www.ruby-forum.com/t/dc-nonexistent-peak-at-transmitter/237258 "2013-11-01T16:25:37Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\
**Post date:** [November 1, 2013, 4:25pm UTC](https://www.ruby-forum.com/t/dc-nonexistent-peak-at-transmitter/237258/1 "2013-11-01T16:25:37Z")

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Hi all,

I am trying to create a complex cosine signal in 100kHz and transmit it  
with USRP in 110MHz using WBX daughterboard. I have observed this signal  
in an oscilloscope and a spectrum analyzer. It can be seen that there is  
a nonexistent peak at transmit frequency of USRP. I also added the  
related photos to be clear.

Any help would be kindly appreciated.

Sema

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**Author:** ![sema](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/35a633/32.png) [@sema](https://www.ruby-forum.com/u/sema)\
**Post date:** [November 1, 2013, 5:05pm UTC](https://www.ruby-forum.com/t/dc-nonexistent-peak-at-transmitter/237258/2 "2013-11-01T17:05:59Z")

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Hi Sema,

You are most likely seeing the local oscillator at the centre due to the  
WBX being a direct conversion daughterboard:

> **[Direct-conversion receiver](https://en.wikipedia.org/wiki/Direct-conversion_receiver)**
>
> A direct-conversion receiver (DCR), also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design that demodulates the incoming radio signal using synchronous detection driven by a local oscillator whose frequency is identical to, or very close to the carrier frequency of the intended signal. This is in contrast to the standard superheterodyne receiver where this is accomplished only after an initial conversion to an intermediate frequency.
> The simplification of performin...

To remove this, you can put the following in the center frequency box of  
the UHD source/sink block in GRC:

uhd.tune\_request(uhd\_center\_freq,  
rf\_freq=(uhd\_center\_freq+uhd\_lo\_offset),rf\_freq\_policy=uhd.tune\_request.POLICY\_MANUAL)

The ‘uhd\_center\_freq’ and ‘uhd\_lo\_offset’ need to be specified. Set  
‘uhd\_lo\_offset’ to be something like ((samp\_rate/2)\*1.1).

Regards,

Mike

–  
Mike J. M0MIK BSc MIET  
Ettus R. Technical Support  
Email: removed\_email\_address@domain.invalid  
Web: [http://www.ettus.com](http://www.ettus.com) [http://www.ettus.com](http://www.ettus.com)

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<div class="post-metadata">

**Author:** ![sema](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/35a633/32.png) [@sema](https://www.ruby-forum.com/u/sema)\
**Post date:** [November 5, 2013, 8:34pm UTC](https://www.ruby-forum.com/t/dc-nonexistent-peak-at-transmitter/237258/3 "2013-11-05T20:34:13Z")

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Sema -

Also, please be sure you run the calibrations provided by UHD. The TX DC  
Offset calibration addresses exactly what you are talking about.

[http://files.ettus.com/uhd\_docs/manual/html/calibration.html](http://files.ettus.com/uhd_docs/manual/html/calibration.html)

Cheers,  
Ben